In velocity prediction programs for yachts the problem of modelling sail forces is greatly complicated by the fact that the sail shapes are not fixed. For a given sail plan, alterations in sail trim produce a wide range of combinations of lift, drag and heeling moment. While the original solution to this problem using the well-known parameters flat and reef has been very successful, it has some obvious defects. In particular, it does not correctly model the practice of twisting a sail to reduce heeling moment. This paper therefore sets out an improved model which satisfies the essential requirements for upwind sails; namely that the model is based upon fundamental aerodynamic principles, allows all the unknowns to be determined from tests (full scale or wind tunnel) and permits the best sail trim to be selected for optimization of performance. The essential new step is to introduce a new trim parameter twist which correctly accounts for the practice of allowing the head of a sail to twist off in order to reduce heeling moment.

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